The present invention relates to an apparatus and method for detecting low refrigerant charge in a refrigeration system.
Automotive air conditioning (A/C) systems employ a mixture of refrigerant and oil, hereinafter referred to as refrigerant charge. A compressor receives necessary lubrication from the oil in the refrigerant charge and circulates the refrigerant charge through the system's condenser, expansion device and evaporator in a refrigeration cycle. Should refrigerant charge drop in volume due to a system leak, improper service or otherwise, the compressor may not receive sufficient lubrication and will eventually become damaged if allowed to operate. It is recognized, therefore, that low refrigerant charge detection is desirable as part of an air conditioning system which is responsive thereto to terminate the system operation prior to the compressor becoming damaged. The compressor may be driven internally by an electric motor or by an automobile engine and coupled thereto via an electromagnetic cycling clutch which engages the compressor to the driving source. A low refrigerant charge typically causes disengagement of the cycling clutch to prevent compressor damage.
There are various known devices or schemes for detecting an insufficiency of refrigerant charge in an operating refrigeration system. Based on specified parameters for various types of systems, the operating compressor is disengaged if the refrigerant charge is determined to be too low. Some detection plans are specific for either fixed or variable displacement compressors while others work for either type of compressor. One scheme for an automotive air conditioning system with an engine driven, fixed displacement compressor uses a load signal (ambient temperature), two capacity signals (evaporator temperature and vehicle speed) and vehicle-specific calibrations to predict the refrigerant charge level and to disable the compressor when the predicted charge is below a predetermined level. Another monitors refrigerant temperature upstream and downstream of the expansion device and checks for a differential value within a predetermined acceptable range. One plan for automotive air conditioning systems with an engine driven, variable displacement compressor requires measuring the response time of the low side pressure as displacement is changed from minimum to maximum. In another detection plan, a mechanical detection device located at the evaporator outlet is triggered by the combination of refrigerant pressure and ambient air temperature to determine low charge while the system is operating. Another method provides for detecting evaporator pressure and temperature and disengaging the clutch if the pressure is too low and the temperature is too low or too high as compared to predetermined fixed values. One embodiment of this method checks if the amount of superheat is too high by comparing the saturation temperature based on the measured refrigerant pressure to the measured temperature. If the measured temperature is too high and the measured pressure is too low, the clutch is disengaged.
One scheme, for either fixed or variable displacement compressors, measures refrigerant temperature at the inlet and outlet of the evaporator and monitors the difference. This scheme also looks at variations in evaporator refrigerant temperature between when the compressor is off and when the compressor is on. Another plan monitors evaporator inlet and outlet temperatures and compressor body temperature while the system is running. Very low charge is detected when the difference between the refrigerant temperatures is large and the body temperature is high.
The above described low refrigerant charge detection plans generally require the system to be operating to obtain most measurements. However, if the refrigerant charge is very low, there is a potential of compressor damage if the compressor is started when the low charge condition exists.